| Several companies offer Doorbell
Alerting systems. Some systems are wireless, some require simple wiring. All
have at least two pieces to them. Some work with existing doorbell systems and
some don't need an existing doorbell system. The wireless systems tend to cost
more. And the better wireless systems avoid false alerts through better
circuitry. Some companies are Ultratec, Sonic Alert, Global Assistive and Silent
Call.
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